The present invention relates to an optical system for an image projection apparatus using a reflective image-forming element.
Japanese Patent Laid-Open No. 2000-206463 has disclosed an image projection apparatus which uses a reflective image-forming element such as a reflective liquid crystal panel and in which its optical system makes convergent angles of a light flux illuminating the image-forming element different from each other in two cross-sections perpendicular to each other to improve contrast of projected images.
This optical system makes the convergent angle of the light flux small in a direction in which illumination light or projection light is bent by 90 degrees (that is, a certain direction) by a polarization beam splitting film of a polarization beam splitter which is used for introducing the illumination light to the reflective image-forming element and analyzing a polarization state of the projection light. Such an optical system suppresses variations of characteristics of the polarization beam splitting film depending on incident angles and reduces the amount of so-called leakage light from the polarization beam splitting film to improve contrast of projected images.
A too-small convergent angle of the light flux of the illumination light or projection light in the above certain direction reduces the amount of the projection light, resulting in dark projected images. The optical system disclosed in Japanese Patent Laid-Open No. 2000-206463 limits the light flux of the illumination light with an aperture stop having different aperture widths in two directions perpendicular to each other. This optical system, however, needs a narrow aperture width of the aperture stop in order to reduce the convergent angle of the light flux, resulting in a significant reduction of the amount of light.
Japanese Patent Laid-Open No. 2001-83604 has disclosed an optical system which reduces the convergent angle of the light flux with a function of a lens such as a cylindrical lens. Such an optical system has been disclosed also in Japanese Patent Laid-Open No. 2000-137290.
Specifically, an afocal system is provided which is constituted by a lens having a positive optical power (+) and a lens having a negative optical power (−), only in a certain direction of an illumination optical system. This afocal system can compress the light flux to −/+|, and converging the compressed light flux with a lens can reduce the convergent angle of the light flux only in the certain direction.
In the optical system disclosed in the Japanese Patent Laid-Open No. 2000-137290, it can be considered that there is no loss of the amount of light because the light flux is compressed. However, since an actual illumination light flux includes not only parallel light rays but also light rays of various directions, angular components of the illumination light flux increase in reverse proportion to the value of −/+|. The increase of the angle of the light flux causes reduction of efficiency at a lens array that produces plural light fluxes for superposingly illuminating the image-forming element and at a polarization conversion element that converts non-polarized light from a light source into predetermined linearly-polarized light, resulting in reduction of the amount of light in practice.